Adding new user in Linux - useradd and adduser not working

I'm trying to create a new user on my Web server. I think it's running Ubuntu but I'm not sure. I've tried useradd and adduser, while working as root, and it keeps telling me "bash: useradd: command not found"

I'm using putty to remote in to the server.

The man pages have information on the useradd command, even if the command doesn't seem to work.

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